1. Field of the Invention
The present invention relates to a data separating circuit for separating digital data inserted in a video signal. More particularly, the present invention relates to a data separating circuit applicable to a television having a teletext system, or a television or video cassette recorder (VCR) having a captioning function, which is adapted by controlling a slicing level, used for converting analog teletext information into digital data, in response to noise or a signal transformation.
2. Description of the Prior Art
A conventional circuit for separating digital data in a video signal employs only analog circuits. In order to generate an average value of a clock synchronous signal of digital data which operates as a data slicing level, a condenser is charged by means of a low pass filter to produce a direct current (DC) value. This value serves as a reference voltage for a comparator to separate digital data which is applied to one input terminal of the comparator. However, characteristics of analog circuit devices are easily affected according to changes in environmental temperature. Furthermore, since the analog devices have wide error tolerances, each system comprised of these analog devices has differing problems due to differences in the analog devices' characteristics.
Data separation in a conventional data separating circuit is performed by clamping the slicing level to a simple DC value. Accordingly, such a system cannot operate properly to transform input signals in the presence of noise. Therefore, conventional data separating circuits have different data separating capacities for each system, and the data separating characteristics deteriorate over time.
Japanese Patent Laid-open Publication No. 90-148986 discloses a technique preventing the disorder of a character screen produced by noise that exceeds the slicing level by selectively passing signals for one line of slice ending and regenerative clock signals. That is, according to this technique, the constituent parts of frequency signals half the clock frequencies included in the signals for one line of slice ending are detected to detect the appearance of the synchronous part arranged in the lead of the data line.
The signal spliced and regenerative clock signals are selectively passed as long as with a period of time from the detecting time to the appearing time of the next horizontal synchronous signal, which can prevent the disorder of the character screen by the false character data provided by the noise that is beyond the slicing level.
In such a system, the data may be exactly separated but cannot cope with the transformation of input signals and noise.